HOW CAN SOME SOIL BE OLDER THAN OTHER SOIL?

Lava laid down by volcanoes eventually develops into soil through the action of plants and weather, and the passage of time. Photo by Simon_sees, http://www.flickr.com.

Question: I just read something that said Africa south of the Sahara has some of the “oldest soils in the world.” But I thought all Earth’s land was once one big continent. So, how can some soil be older than other soil?

Answer: The earth’s land was indeed one large super-continent, Pangaea, about 300 million years ago in the Paleozoic. So why aren’t all soils the same age in our current geologic time…the Holocene?

The short answer is: climate, water, slope, volcanoes, and time. The oldest soils on the planet tend to be located where the landscape has been stable through geologic time, allowing the time necessary to “age” the soil through weathering. The earth’s oldest soils, in other words, are those in which weathering has been going on uninterrupted for the longest periods.

Many soils are considered younger because they formed more recently from rock. This happens when freezing/thawing, weathering, and vegetation combine to form what’s called “unconsolidated material” from rocks. In that material, a new soil begins to form.

Or…on many occasions through geologic time a good part of the world’s land mass was covered by glaciers. The ice ground up the existing soil and rock and left behind soil material that started to develop all over again. The last glaciation ended only about 10,000 to 20,000 year ago. Soils subsequently formed in glacial till, glacial outwash, and other common glacial landscape features. You would expect to find older, more weathered soils closer to the tropics, where glaciation did not occur.

New soil material is being deposited on the earth’s surface all the time. In a river’s floodplain, new soil material is being deposited and young soils are being formed. When a volcano erupts, new material is also laid down and soil development starts. In fact, entire islands are forming from volcanic material as we speak. Our 50th state, Hawaii, is adding new territory and young soils through volcanic activity.

The result of all these actions is a landscape that can have young to very old soils next to each other. When you understand how soils develop you can look at the landscape and predict where the younger soils are developing and where the older soils continue to age. From Pangaea to the Holocene, soils contain the natural history of the planet.